FRR is a mechanism to protect against RSVP-TE signaled LSP failures by reacting to these failures as soon as possible. FRR is set up from the iLER, which signals the transit routers to precompute their backup LSPs. FRR creates a precomputed backup LSP from each node in the LSP path. If a link or LSP between two routers fails, traffic is rerouted immediately onto the precomputed backup LSP.
The 7705 SAR supports FRR facility backup and one-to-one backup.
Facility backup mode allows FRR to be enabled on an aggregate basis and protects a whole node or a whole link, regardless of the number of LSPs using that link. In other words, facility backup mode creates a common bypass tunnel to protect all LSP-paths traversing a common facility path. It provides flexibility, faster provisioning, and faster convergence times compared with one-to-one backup or LSP redundancy. One-to-one backup allows FRR to be enabled on a per-LSP basis.
With both methods, MPLS switches build many possible detour routes on the nodes between the ingress and egress nodes of an LSP. The facility backup method creates a detour route between two nodes, called a bypass tunnel, which is a single tunnel that follows the primary LSP path except where the link or node has failed. Traffic then switches to the bypass tunnel. The bypass tunnel merges with the original LSP path at the merge point (MP) as soon as possible. The one-to-one backup method creates a detour route, called a detour LSP, for each LSP that needs to be rerouted. Unlike the bypass tunnel, the detour LSP takes the best path to the termination point, and does not merge with the original LSP as soon as possible. The detour LSPs of a one-to-one backup LSP can merge at a detour merge point (DMP), which can either be at the termination point or at a point along the primary LSP.
One of the major differences between facility and one-to-one backup is the scalability offered by the protection method. In facility backup mode, all LSPs of the same type are rerouted over the bypass tunnel. Hence they are all protected against the failure of a node or link in the network. In facility backup mode, each LSR along the path verifies that it has a bypass tunnel available to meet its requirements; otherwise, if it can, it signals a new bypass tunnel based on the requirements. If a new LSP is configured for FRR facility backup, the existing backup tunnels are scanned and if any one of them can be used for recovery, it is preferred. If there are no common links, then a new bypass tunnel will be signaled, assuming that the LSP requirements can be met. One-to-one backup mode uses similar reroute and protection methods except a detour route is applied on a per-LSP basis.
The 7705 SAR uses CSPF to calculate the explicit route and dynamically signal the FRR LSP.
With facility backup mode, routers check the contents of the Record Route Object (RRO) in the received RESV message to determine the bypass tunnel endpoint in the FRR facility. For link protection, the router uses the RRO to check the IP address of the next-hop router attached to the far end of the link along with the label allocation information and to build the bypass tunnel. This label is preserved until the LSP is merged at the MP. For node protection, the router uses the RRO to determine the next-next-hop router and the label it is expecting. The collection of RRO information is enabled through the record and record-label options.
If, after this process, another LSP requests FRR using the facility backup method, the router checks and compares its session object to the existing session objects and if there is a match, the router binds that LSP to the same bypass tunnel. If there is no match, another bypass is created.